Refactoring of pkcs11_store.c module

[danielinux]

• [x] add unit tests for pkcs11_store.c
• [x] defects detected by unit tests are fixed
• [x] refactoring with a new powerfail-safe design

This new design uses two sectors at the beginning of the keyvault partition. The first sector contains the allocation tables for the nodes in flash, including a bitmap to keep track of the free slots. The second sector is a pre-commit 'backup' sector, used to update the content of the vault in two steps. This guarantees powerfail protection.

All hal_flash_erase / hal_flash_write operations are now on entire sectors. Sector content is cached to memory, then written to the backup sector, then committed to the actual destination sector.

[danielinux]

Awaiting feedback (zd18366)

[danielinux]

Ready to review